Short term not as bad as 9/11, which included a complete economic meltdown which had even worse industry ramifications than the grounding.

But this has the potential to really screw the global economy if it continues for weeks or months unabated. Although I assume they could do some experimenting and come up with some mitigation plans to track the exact extent and severity of the cloud at any given time in order to allow some flights to go. Right now they are canxing everything out of an abundance of caution.